Wireless MAN Components

Components of a wireless MAN generally come in matching pairs because they support fixed wireless connectivity from one point to another. Take a look at the primary components of a wireless MAN.

Bridges

The industry definition of a bridge is a device that connects two networks that might use the same or a different data-link layer protocol (Layer 2 of the OSI reference model). Figure 6-1 illustrates this concept.

Wireless bridges are generally at each end of a point-to-point link, such as those that interconnect two buildings. A bridge has a wired port that connects to the network and a wireless port that interfaces with a transceiver. The bridge receives packets on one port and retransmits them on another port. A bridge will not start retransmission until it receives a complete packet. Because of this, stations on either side of a bridge can transmit packets simultaneously without causing collisions.

Some bridges retransmit every packet on the opposite port regardless if the packet is heading to a station located on the opposite network. A learning bridge, which is more common, examines the destination address of every packet to determine whether it should forward the packet based on a decision table that the bridge builds over time. This increases efficiency because the bridge will not retransmit a packet if it knows that the destination address is on the same side of the bridge as the sending address. Learning bridges also age address-table entries by deleting addresses that have been inactive for a specified amount of time.

The bridges within the network are transparent to users. Packets are sent through the bridge automatically. In fact, users have no idea that their packets are traversing a link leading to a different location.

Bridges Versus Access Points

Access points connect multiple users wirelessly to each other and to a wired network. For example, several users equipped with 802.11 NICs might associate with a single access point that connects to an Ethernet network. Each of these users has access to the Ethernet network and to each other. The access point in this case is similar to a bridge device, but the access point interfaces a network to multiple users. A bridge interfaces only other networks.

It's possible to use a wireless bridge indoors. For example, a wireless LAN bridge can interface an Ethernet network directly to a particular access point. This might be necessary if few devices, possibly in a far-reaching part of the facility, are interconnected through Ethernet. A wireless LAN bridge plugs into this Ethernet network and uses the 802.11 protocol to communicate with an access point that is within range. In this manner, a bridge enables the wireless connection of a cluster of users (actually a network) to an access point.

Basic Ethernet-to-Wireless Bridges

An Ethernet-to-wireless bridge (see Figure 6-2) connects directly to a single computing device through an Ethernet port and then provides a wireless connection to an access point. This makes it useful when the device, such as a printer, PC, or video game console, has an Ethernet port and no wireless NIC. In some cases, you might have no way of adding a wireless NIC, which makes a basic bridge the only way to go wireless. Printers and video game machines are common examples of this scenario.

Workgroup Bridges

Workgroup bridges are the answer for connecting wireless networks to larger, wired Ethernet networks. A workgroup bridge acts as a wireless client on the wireless network, and then interfaces to a wired network. The wired side connects to an Ethernet switch that connects multiple devices. A workgroup bridge offers more robust and higher-end management and security utilities?with higher prices?as compared to a basic bridge.

Directional Antennae

The antenna is an important element of a wireless MAN. Unlike other types of wireless networks, most antennae for wireless MANs use directional antennae, mainly because they operate over wider areas. Figure 6-4 illustrates the propagation of radio waves from a directional antenna. This contrasts with an omnidirectional antenna, which transmits radio waves in all directions.

Different types of antennae have different vertical and horizontal beamwidths. For example, an omnidirectional antenna has a horizontal beamwidth of 360 degrees and a vertical beamwidth that ranges from 7 to 80 degrees. A semidirectional antenna might have a vertical beamwidth of 20 degrees and a horizontal beamwidth of 50 degrees. Generally, the narrower the beamwidth, the longer the range when transmit power is kept constant.

Semidirectional Antennae

There are several different types of antennae that have semidirectional radiation patterns. For example, a directional patch antenna will have at least double the range as compared to an omnidirectional antenna. You can easily mount a patch antenna on a wall on one side of a facility and effectively cover a large area. A Yagi antenna, a common antenna invented by Japanese inventor Hidetsugu Yagi, is the semidirectional antenna best for long-range applications.

Semidirectional antennae effectively increase the signal's amplitude?referred to as gain?by approximately 10 times. Their use is mostly for extending wireless LANs to cover a larger area. For example, a university might employ a Yagi antenna to effectively cover a large, open, outdoor area of the campus. Wireless MANs generally span much greater distances and require greater values of gain.

Highly Directional Antennae

A highly directional antenna has an extremely narrow beamwidth, with long radiation patterns and corresponding range. To achieve this degree of directivity, you need to use dish antennae that focus the radio energy mostly in one direction. These types of antennae are expensive compared to omni- and semidirectional antennae; however, the costs may be feasible if the solution requires long range.

Many of the higher-gain directional antennae use a parabolic dish to focus the radio frequency (RF) power in one direction. A parabolic dish, for example, has a narrower horizontal and vertical beamwidth of 4 to 25 degrees. This extreme focusing of the RF power increases range significantly.

A problem, however, is that the dish antennae are subject to damage from weather because of excessive wind loading, especially if the antenna is not mounted correctly. As a result, highly directional grids that have plenty of holes in the dish are generally safer to deploy.

In addition, both semidirectional and highly directional antennae require a clear line of sight between both ends of the system. In some cases, RF signals can pass through trees and some buildings, but infrared requires an unobstructed path. RF and infrared signals also experience periodic attenuation due to obstructions moving across the path of the signal, such as passing trains and automobiles. Planning wireless MANs is difficult in city environments because of buildings that block the path between the ends of the system.

Effect of Polarization

Antenna polarization is the physical orientation of the antenna along a horizontal or vertical plane. For example, vertical polarization, which is the most common for wireless LANs, occurs when the antenna is perpendicular to the Earth. Parallel polarization applies to an antenna that is parallel to the Earth.

To maximize the transfer of RF energy from the transmitter to the receiver antenna, both antennae should have the same polarization. If one antenna has vertical polarization and the other has horizontal polarization, no transfer of power or communications will occur.